dirstate-v2: Add a zero-size error type for dirstate v2 parse errors
This error should only happen if Mercurial is buggy or the file is corrupted.
It indicates for example that:
* A part of the file refers to another part, and the byte offset or item count
would cause reading out of bounds, beyond the end of the file.
* The byte for an entry state has an invalid value
When parsing becomes lazy, many more functions will return a `Result` with
this error. Making it zero-size reduces the work that the `?` operator needs
to do to pass around the error value.
Differential Revision: https://phab.mercurial-scm.org/D10748
$ hg init a
$ cd a
$ touch a; hg commit -qAm_
$ hg bookmark $(for i in $($TESTDIR/seq.py 0 20); do echo b$i; done)
$ hg clone . ../b -q
$ cd ../b
Checking that when lookup multiple bookmarks in one go, if one of them
fails (thus causing the sshpeer to be stopped), the errors from the
further lookups don't result in tracebacks.
$ hg pull -r b0 -r nosuchbookmark $(for i in $($TESTDIR/seq.py 1 20); do echo -r b$i; done) -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" ssh://user@dummy/$(pwd)/../a
pulling from ssh://user@dummy/$TESTTMP/b/../a
abort: unknown revision 'nosuchbookmark'
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